The present invention relates to an ink-jet printer for printing an image on a printing medium by discharging ink from an ink-jet printhead onto the printing medium and a control method and apparatus for the ink-jet printer.
In general, an ink-jet printer capable of outputting color images has a plurality of printheads for discharging inks of four colors, i.e., yellow, cyan, magenta, and black (to be respectively referred to as Y, M, C, and K hereinafter). Recently, an arrangement for printing color images by using six or more inks including light inks obtained by decreasing the densities of color inks (e.g., light cyan and light magenta (to be respectively referred to as LC and LM hereinafter) inks obtained by decreasing the densities of cyan and magenta inks) and inks having normal densities is often used to suppress the graininess of dots in a highlight portion.
A printhead used for an ink-jet printer will be described next.
Ink-jet printing schemes include a scheme using an electrothermal transducer (heater) as an element for generating discharge energy to discharge ink droplets and a scheme using a piezoelectric element. In both schemes, ink is discharged by supplying an electrical signal to an element for generating discharge energy. An advantage of the former scheme is that only a small space is required to place each heater serving as an element for generating discharge energy. This makes it possible to simplify the arrangement of an ink-jet printhead and hence reduce its size. In addition, it is relatively easy to achieve an increase in density.
A disadvantage of this scheme is that the heat generated by each heater is accumulated in the printhead. As a consequence, discharged ink droplets tend to vary in volume. In addition, shock (cavitation) is produced when air bubbles generated to discharge ink shrink and disappear to greatly affect the heaters.
As methods of solving these problems, for example, ink-jet printing methods and ink-jet printheads are disclosed in Japanese Patent Laid-Open Nos. 54-161935, 61-185455, 61-249768, and 4-10941. According to these methods and printheads, each ink-jet head has orifices for discharging a liquid, ink channels, each of which communicates with the orifice and is filled with ink, and an electrothermal transducer placed in each ink channel. This electrothermal transducer is generally formed by a thin resistive element. According to a characteristic feature of this electrothermal transducer, a pulse-like current is supplied (application of a driving pulse) to the electrothermal transducer through an interconnection to generate heat energy. By using such a printing method, the stability of the volume of each ink droplet can be improved, and small droplets can be discharged at high speed. This makes it possible to improve the durability of each heater by solving the problem caused by cavitations produced by printing and defoaming of ink.
A plurality of printheads are used to print images in the above four colors, Y, M, C and K, or six colors, Y, M, C, K, LC and LM. Such printheads are mass-produced. The ink discharge amounts of the respective printheads vary due to manufacturing variations in characteristics of the respective printheads. These variations roughly amount to, e.g., xc2x110% with respect to a standard discharge amount. Since the ink discharge amounts of the respective printheads vary in this manner, printed images differ in their density and color appearance.
In general, a printer is designed such that the tone of an output image is determined on the premise that each printhead has a standard ink discharge amount. For this reason, the image printed by using an ink-jet printer using a printhead whose ink discharge amount deviates from the standard discharge amount differs in tone from the designed color image. With an improvement in image quality in recent ink-jet printers, an image with photo quality can be obtained. For a photographic image, its color tone is an important factor that determines its image quality. If, therefore, the tone deviates from the designed value for the above reason, the following problems arise:
(a) The color reproducibility deteriorates.
(b) Tone skip occurs (due to an imbalance between normal ink and light ink of the same color, in particular), and the tone continuity deteriorates.
(c) False contours are produced.
These problems may greatly impair the image quality.
For this reason, a method is disclosed in Japanese Patent Laid-Open No. 2-167755, in which xcex3 correction tables used for xcex3 (gamma) correction in an image processing unit are prepared for the respective ink discharge amount characteristics of printheads, and the ink discharge amount characteristic information written in the printhead mounted in an ink-jet printer is read out to select a xcex3 correction table corresponding to the discharge amount information, thereby performing image processing for the corresponding image data. With the use of such a method, even if the ink discharge amounts of printheads vary, changes in the density and color appearance of images can be minimized.
A characteristic feature of the above conventional printhead is that it has excellent ink droplet discharge stability. If, however, the driving frequency of the printhead increases and printing operation is continuously performed, or an image with a high duty is printed, the temperature of the printhead rises, resulting in variations in the ink discharge amount of the printhead. The variations reach about 15 to 20% due to a rise in temperature. This is because, an air bubble generated by the heater considerably grows with a rise in temperature, and consequently, the remaining ink in the nozzle liquid chamber decreases at the time of ink discharging operation, resulting in an increase in the amount of ink droplet discharged at once. The adverse effects caused by such an increase in ink discharge amount include an increase in graininess due to an increase in dot diameter, a change in print density, and beading (ink soaking and smear due to ink soaking) caused when the amount of ink supplied to a printing media exceeds its ink absorbing (receiving) capacity. Of these adverse effects, beading is especially serious. The occurrence of such beading causes information loss in a high-density portion of an image, or considerably degrades a sense of resolution, resulting in serious problems.
According to Japanese Patent Laid-Open No. 2-167755 that discloses the method of performing image processing upon selecting a xcex3 correction table corresponding to ink discharge amount information, only a technique of making density differences due to variations in ink discharge amount become constant regardless of whether the temperature of a head rises or not, is disclosed. However, this method cannot prevent beading caused when the ink discharge amount increases with a rise in head temperature.
The present invention has been made in consideration of the above conventional techniques, and has as its object to provide an ink-jet printer which prevents ink soaking on a printing medium due to a rise in head temperature by setting the ink discharge amount of an ink-jet head, which increases with a rise in the temperature of the ink-jet head, within the ink receiving capacity of the printing medium, and a control method and apparatus for the ink-jet printer.
It is another object of the present invention to provide an ink-jet printer which improves the quality of a printed image by storing information about variations in the ink discharge amount of each printhead in the memory of each printhead and changing image processing for generating print data in accordance with the information of each printhead, and a control method and apparatus for the ink-jet printer.
It is still another object of the present invention to provide an ink-jet printer which can prevent beading caused by an increase in ink discharge amount with a rise in the temperature of each printhead, and a control method and apparatus for the ink-jet printer.
It is still another object of the present invention to provide an ink-jet printer which stores, in a printhead, ink discharge amount correction information based on dimensional tolerances in the process of manufacturing the printhead, and prints an image by generating print data on the basis of the correction information, and a control method and apparatus for the ink-jet printer.
In order to attain the above-described objects, an ink-jet printer of the present invention comprises the following structure.
An ink-jet printer for printing by discharging ink from an ink-jet head onto a printing medium, the apparatus comprises: read means for reading out head information about an ink discharge amount of the ink-jet head from a memory in which the head information is stored; selection means for selecting a correction table for correcting image data on the basis of the head information read out by said read means; image processing means for generating print data by processing the image data by using the correction table selected by said selection means; and control means for controlling to print an image on a printing medium on the basis of the print data generated by said image processing means, wherein the correction table stores correction data for the image data which is based on a print density on the printing medium and an ink receiving capacity allowed in printing on the printing medium in correspondence with an ink discharge amount characteristic of the ink-jet head.
In order to attain the above-described objects, a control method for an ink-jet printer of the present invention comprises the following steps.
A control method for an ink-jet printer for printing by discharging ink from an ink-jet head onto a printing medium, comprising the steps of: receiving head information about an ink discharge amount of the ink-jet head mounted on the ink-jet printer from the ink-jet printer; selecting a correction table for correcting image data on the basis of the head information received in said receiving step; processing the image data by using the correction table selected in the selection step to generate print data; and transmitting the print data generated in said processing step to the ink-jet printer, wherein the correction table stores correction data for the image data which is based on a print density on the printing medium and an ink receiving capacity allowed in printing on the printing medium in correspondence with an ink discharge amount characteristic of the ink-jet head.